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Runoff directional dataset

Deep time experiments typically require significant modification of the land/sea mask, and therefore must remap river networks across the paleo topography by creating a new runoff directional dataset. This dataset is used by the River Transport Model (RTM) to route river runoff to the ocean.

RTM runs inside the land model, but uses a fixed regular grid that is different from the parent CLM grid (the CLM and CAM models use the same grid, which for CESM1, is a Gaussian grid, typically at T31 resolution for deep time), or a finite volume grid (typically FV4x5 for deep time). The runoff directional forcing file required for RTM is an ASCII file containing latitude, longitude, and an integer value describing the vector (direction) for runoff flow at each RTM grid point. The integer values are numbered from 1 to 8: 1=N, 2=NE (45o), 3=E (90o), etc. See Figure 6 for complete vector directions and integer labels.

The CESM1 default RTM grid is at 0.5 degree resolution (rdirc05). However, deep time modelers typically use a 2x2 degree grid.

Creating RTM forcing files [rdirc.csh/topo2rdirc.F90]

The script rdirc.csh computes the runoff direction at each land grid point based on the user-provided topobathy.nc file containing topography and ocean bathymetry. An ASCII file of runoff vectors is created which is used as input to the RTM at runtime. This tool uses topography to compute the direction of runoff flow. The output filename from rdirc.csh for your RTM forcing file is simply fort.10 (Fortran output file), so you may wish to rename your RTM forcing file to something more descriptive. For example, the default RTM forcing file for present day is called rdirc.05. An example filename for a paleo run could be: rdirc_myrun.resolution.date.

Correcting RTM forcing file [check.csh/check_inf_loop.F90]

If your surface topography has any internal basins or large flat regions, infinite loops will result, and rdirc.csh will produce another ASCII file with these loops (fort.11). An infinite loop is a region from which runoff will never flow out to the coastline, but circulate back to the starting point (). If infinite loops are not removed, global freshwater will not be conserved, and undesirable trends in global volume averaged ocean salinity may result. Using a plot of runoff vectors and the list of infinite loops, you must hand edit the runoff vector file to remove all infinite loops. A tool to check for your infinite loops is called check.csh and is included in setup_tools. Feel free to use your own tool to edit the RTM forcing file. The shaded section of Figure 6c shows an example of an infinite loop.

shell script

check.csh

source code

check_inf_loop_sed.F90

inputs

fort.10

output

fort.11 (infinite loops locations)

Plotting the vectors on a map [plotdirc.csh/rdirc.ncl]

An NCL script to plot your RTM forcing file vectors onto a latitude/longitude map is included in the setup_tools tar file. An example plot of a RTM vector directional map generated using plotdirc.csh is shown in Figure 7.

shell script

plotdirc.csh

source code

rdirc.ncl

inputs

fort.10

output

postscript file for vector plots

Iterate

Creating the RTM forcing file and checking for infinite loops is an iterative process. You will need to repeat steps 4.3.4.2 and 4.3.4.3 as many times as needed until you have a clean RTM forcing file. You may want to save your original fort.10 for comparison with later iterations.